PROJECT 2: SUMMARY Immunotherapy carries great promise for the treatment of several solid tumors, including malignant glioma. But in order to have a true impact in the disease course, a combinatorial strategy is likely required. In particular in glioblastoma, a heterogeneous disease characterized by local immunosuppression and an immune-privileged environment as well as increased vascularity, a combination of radiotherapy with concomitant immunomodulation by both a checkpoint inhibitor and IDO inhibitor could prove effective. In a series of preclinical experiments, we have shown that the triple combination of radiotherapy, checkpoint inhibition and IDO inhibition exert enhanced antitumor activity. Strikingly, this triple therapy led to a durable survival benefit in mice with large, intracranial GBM. We confirmed that this approach inhibited immunosuppressive IDO1 metabolism, as determined by an increase of intratumoral tryptophan and a decrease of kynurenine levels. These preclinical findings are now to be translated within a clinical trial to patients suffering from newly diagnosed GBM. The innovation of this trial is enhanced by the incorporation of ?-[11C]-methyl-L-Trp (AMT)-PET imaging, which is a noninvasive method to quantify tryptophan uptake and/or IDO1 enzyme activity, and a comprehensive immunmonitoring panel. AMT-PET imaging therefore facilitates the real-time evaluation of intracranial IDO1 enzyme inhibition, which is advantageous to patients with tumors that are not conveniently accessible for repeat tissue sampling. The clinical trial will include correlative analysis of systemic and intratumoral immunological markers, tumor gene expression and mutagenic burden, quantitative AMT-PET data, and treatment outcome (overall survival as primary endpoint). In parallel, we will conduct further experiments with mice reconstituted with human immune systems and inoculated with tumors of human origin (PDX). This will allow for testing hypotheses arising from the clinical experience, as well as additional therapeutics that could be incorporated in future clinical trials that are derivative of that being tested in patients here. Ultimately, our research will influence future immunotherapeutic strategies in the multimodality management of patients with glioma.